Method of forming tubular mica capacitor

ABSTRACT

This disclosure relates to the art of electric capacitors of the mica type and more particularly relates to a method of forming a capacitor comprising a rigid tubular member of insulating material in which is positioned a substantially rectangular mica capacitor section that is retained in fixed position with terminal leads protruding from opposed ends of the tubular member by filling the latter with a thermosetting resin.

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[45] Patented May 25 1971 t [73] Assignee Aerovoii Corporation Pm-naryE'mmller-John F' Campbell New Bedford Mass Assmanl Exammer-RlchardBernard Lazarus Attorney-Arthur B. Colvin [54] METHOD OF FORMING TUBULARMICA CAPACITOR 2 Claims, 3 Drawing Figs. [52] 29/25A2,

mosetting resin.

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ATTORNEY METHOD F FORMING TUBULAR MICA CAPACITOR This application is adivision of copending application Ser. No. 743,787, filed July l0, |968now U.S. Pat. No. 3,522,493.

As conducive to an understanding of the invention, it is noted that inmany cases because of their operating characteristies, mica capacitorsare desired. Such capacitors having a long life and a high safetyfactor.

However, due to the fact that it is preferred because of their superiorelectrical characteristics, to use natural mica sheets as t`nedielectric of the capacitor, since these sheets are ex tremely brittle,they cannot be rolled and hence capacitors which use natural mica sheetsas the dielectric are generally rectangular in shape and substantiallyin the form of a parallelapiped.

Since, in the present state of technology, automatic material handlingmachines are best able to feed tubular members, the rectangular form ofthe conventional mica capacitor renders them difficult to be handled andfed automatically at a high rate of speed,

Where attempts are made to encapsulate the mica capacitorsection bymolding, so that the resultant configuration will be tubular, sincemolding techniques require the exertion of a high degree of pressure,lthe relatively brittle mica plates or sheets often tend to crackthereby rendering the capacitor useless for its intended purpose.

Furthermore", due to the pressure exerted in the molding operation, theinternal mica section tends to shift with the result that it is notuniformly enclosed by the external molding material and in many casesthere is a relatively thin layer of molding material on one side or endof the capacitor ysection with the result that it is susceptible tohumidity, therefore affecting the effectiveness of the capacitor.

In addition to the foregoing, under the pressure exerted during themolding operation, the capacitor section tends to bow and where the endsof the section are retained together by crimping, such crimped ends maybe opened with resultant failure of electrical connections.

Furthermore, the bowing of the capacitor section under molding pressuremay change the characteristics of the capacitor which is undesirable. Inaddition to the foregoing, where foil is interposed between adjacentlayers of mica, the use of pressure during molding may cause the foil tomove away from the mica with resultant high resistance connection.

In addition, the molding powder can be forced.. laterally between themica sheets under the relatively high pressures used in molding, therebycausing cracking of the mica and breakdown of the capacitor.Furthermore` it is necessary that the terminal leads extending from thecapacitor section be carefully aligned in grooves in the mold in orderthat they extend axially from the molded unit to permit subsequentautomatic handling thereof. This procedure is time-consuming since itrequires each section to be individually handled and often the leads arenot straight which is also undesirable.

Where the mica capacitor sections are dipped into a suitable coatingmaterial in order to cover the same, it is diicult to form a uniformlycylindrical unit and the relatively thin walls at the ends ofthecapacitor break off as the capacitor leads are handled, possibly openinga path for moisture into the capacitor section.

It is accordingly among the objects of the invention to provide atubular mica capacitor which may readily be manufactured at low costwith assurance that the unit will be truly cylindrical in configurationwith terminal leads extending axially therefrom and without anypossibility of excessive pressure being applied to the mica sectionduring fabrication of the tubular capacitor which might cause crackingof said mica with resultant failure of the unit.

According to the invention, these objects are accomplished by thearrangement and combination of elements hereinafter described and moreparticularly recited in the claims.

ln the accompanying drawings in whichis shown one of various possibleembodiments of the several features of the invention;

FIG. l is a longitudinal sectional view of a tubular mica capacitoraccording to the invention, and

FIGS. 2 and 3 are diagrammatic views illustrating steps in themanufacture of said tubular capacitor.

Referring now to the drawings, as shown in FIG. 1, the mica.. capacitorcomprises a cylindrical casing or tube 1l of suitable'- insulatingmaterial such as polyethylene. The tube l1 is of internal diametergreater than the width of the mica capacitor section 12 to be positionedtherein and is of length greater than the length of said section l2.

As shown in FIG. 1, the capacitor section l2 comprises a plurality ofsuperimposed mica sheets 13 each of which has a silver coating 14applied to opposed surfaces thereof with an insulating plate 15 beingpositioned on opposed faces of the superimposed mica sheets. Alternatesilveredsurfaces 14 have foil electrodes 16, 17 secured thereto, saidelectrodes being so arranged at each end 18, 19 of the capacitor section12 as to connect alternate plates in parallel. The silvered mica platesare clamped together by means of suitable metal bands or straps 20, 2lcrimped around the ends 18, 19 of the capacitor section and alsoencompassing the foil electrodes 16, 17 atv said ends. Terminal leads 22and 23 connected at one end respectively to each of said bands 20, 2lextend outwardly from the ends of said capacitor section 12, said leadsbeing longitudinally aligned as shown.

The capacitor section is positioned in the bore of the tube 11 andretained in position so that the terminalleads extend axially outwardlyin opposed direction from said tube 11. Thereupon, the tube 11 is filledwith a suitable insulating material such as an epoxy resin 24 tocompletely encapsulate the capacitor section l2 on its sides and endswith a uniform thickness of resin along the length of the section 12.

According to one illustrative embodiment of the invention, to assemblethe capacitor, a suitable jig 31 is provided as shown in FIG. 2 whichcomprises a plate 32 that has a cylindrical boss 33 of external diameterjust slightly less than the internal diameter ofthe insulating tube 1lso that the end 34 of-the latter may encompass said boss 33 and beretained in upright position. The boss 33 has an axial bore 35 intowhich the terminal lead 23, for example, may be positioned with the end18 of the capacitor section l2 resting on the top surface 36 of saidboss. The height of boss 33 is such that when the end 18 of thecapacitor section rests thereagainst the other end 19 will be spacedfrom the end 39 of the tube by the same distance that end 18 is spacedfrom end 34 of the tube. Thus, the capacitor section is longitudinallycentered in the tube for the reason hereinafter set forth. A suitableclamp 37 compris ing for example, two horizontally movable members 38are adjusted to clamp the terminal lead 22 therebetween, said terminallead 22 protruding beyond the upper end 39 of the tube 1l. Thus, byreason of the axial bore 35 in the jig 3l and the.

clamping members 38, the terminal leads 22, 23 are yretained invertically aligned position axially of tube ll, with each of thesidewalls of the capacitor section uniformally spaced along their lengthfrom the wall of the tube and with the capacitor section l2longitudinally centered in the tube 1l.

Thereupon, the upper end of tube ll is filled with a suitablethixotropic resin 24 kwhich will form a cap 24 in the upper end.

39 of tube 11. Such a resin is used to prevent flow, down the tube whichcould fill the bore 35 and thus prevent removal of the lead 23therefrom. In addition, it is a characteristic of such a resin to levelout at its outer surface so that there is assurance that the end 39 ofthe tube 1l will be completely sealed. The resin is then cured so thatit will set.

The capacitor unit is then removed from the jig 31 and inverted andpositioned on a second jig 4l. This second jig which is shown in FIG. 3comprises a cylindrical holder 42 rising from a surface 43, said holderbeing of inner diameter just slightly less than the outer diameter ofthe end 39 of tube ll so that it may retain the latter in uprightposition, said surface having a bore 44 therein axially aligned withrespect to said cylindrical holder 42 to accommodate said terminal lead22. A clamp 37" similar to clamp 37 is used to hold the lead 23 in axialalignment with lead 22. To complete the assembly of the capacitor, it ismerely necessary to fill the interior of the tube 11 with a suitableresin and cure the same.

As the result of the method above-described, the capacitor section 12will be completely encapsulated in the epoxy resin 24 and retained inposition in the tube 1l with the terminal leads 22 and 23 extendingaxially from the tube ll in opposed direction and with the entire sidesand ends of the section 12 completely encapsulated and with a relativelythick uniform layer of resin 24 between the ends of the section and theends of the tube which insures that no moisture will penetrate throughthe ends of the tube l1.

With the method above-described, a completely encapsulated tubular micacapacitor is provided without the application of any molding pressurebeing exerted against the capaci- 4 tor section and with assurance thatthe terminal leads will extend axially from opposed ends of theassembled unit and with further assurance that the capacitor sectionwill have relatively thick layers of insulating material between theends thereof and the exterior.

As many changes could be made in the above method and construction, andmany apparently widely different embodiments of this invention could bemade without departing from the scope of the claims, it is intended thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

Having thus described our invention, whatwe claim as new and desire tosecure by Letters Patent of the United States is:

We claim:

l. The method of forming a tubular mica capacitor having a substantiallyrectangular capacitor section with oppositely extending terminal leadsand a tubular outer casing, which comprises the steps of positioning oneend of the tubular casing on an associated cylindrical boss so as torise vertically therefrom, positioning the capacitor section completelyin the casing with the lower one of the terminal leads of the capacitorsection in an axial opening in the boss, supporting the other terminallead of the capacitor section extending from the upper end of thetubular casing so that the two terminal leads are vertically aligned,capping the upper end of the tubular casing with a suitable thixotropicresin, curing said resin, then inverting the capped tube and supportingthe latter in upright position and filling the interior of the tube witha suitable insulating material to encapsulate the capacitor section andthen curing the same.

2. The method set forth in claim I in which the terminal lead extendingfrom the capped end of the capacitor is positioned in a vertical borethat is axially aligned with a cylindrical holder, the latterencompassing the end of the tube to v retain the latter in verticalposition.

1. The method of forming a tubular mica capacitor having a substantiallyrectangular capacitor section with oppositely extending terminal leadsand a tubular outer casing, which comprises the steps of positioning oneend of the tubular casing on an associated cylindrical boss so as torise vertically therefrom, positioning the capacitor section completelyin the casing with the lower one of the terminal leads of the capacitorsection in an axial opening in the boss, supporting the other terminallead of the capacitor section extending from the upper end of thetubular casiNg so that the two terminal leads are vertically aligned,capping the upper end of the tubular casing with a suitable thixotropicresin, curing said resin, then inverting the capped tube and supportingthe latter in upright position and filling the interior of the tube witha suitable insulating material to encapsulate the capacitor section andthen curing the same.
 2. The method set forth in claim 1 in which theterminal lead extending from the capped end of the capacitor ispositioned in a vertical bore that is axially aligned with a cylindricalholder, the latter encompassing the end of the tube to retain the latterin vertical position.